Popular comes from the Latin adjective populāris “pertaining to all or most of the people, belonging to or used by the common people (as opposed to the military, the aristocracy, or the senators)”; it is a very loaded word in Roman political history.
Populāris is a derivative of the noun populus “a human community, nation, the members of a society,” and in Rome “the entire people exercising its full legislative and judicial authority” (another weighty word). It is surprising that there is no certain etymology for populus. The most likely of several possible etymologies derives populus from Etruscan puplu (Etruscan, an extinct ancient language, is the “go to” language for Latin etymological problems); puplu appears in the name of the Etruscan town Pupluna ( Populōnia in Latin). For good measure, Rōma, the name of the city, is named after an Etruscan family, as are three of Rome’s seven hills.
The current, most familiar sense of popular , “regarded with favor, approval, or affection by many people,” dates from the very early 17th century.

Did you mean simple?